A femto base station or a so-called home base station has recently attracted much interest in wireless industry. Standardization process for home base stations is on going in 3GPP for both Evolved Universal Terrestrial Radio Access Network (E-UTRA) and IEEE 802.16. For UTRA, the home base station was standardized in release 8. However, in both UTRA and E-UTRA advanced features related to the home base station such as mobility procedures, interference management and home base station control etc are also being introduced for future releases. Femto base stations are already operational in other technologies such as GSM and 3GPP2 CDMA technologies (e.g. CDMA2000 1xRTT and High Rate Packet Data (HRPD)).
A femto base station e.g. Frequency Division Duplex/Time Division Duplex (FDD/TDD) home NodeB, TDD/FDD home eNB, GSM home BS, CDMA2000 1x home BS, HRPD home BS, IEEE 802.16 home base station or access point etc may be deployed at home or public/private premises such as shopping malls, office buildings, etc. A femto base station may share the same carrier with other macro/micro/pico base stations i.e. non femto base stations, or alternatively be assigned a dedicated carrier only for its operation. In the former scenario, femto base stations may generate unnecessary high interference to surrounding macro base stations. Therefore, the transmit power of the femto base station needs to be properly regulated and controlled.
In the legacy UTRAN specifications, three classes of base stations are defined, namely a wide area BS that serves macro cell deployment, a medium range BS that serves micro cell deployment and a local area BS that serves pico cell deployment i.e. smaller cells.
In E-UTRAN specifications two classes of base stations are defined, namely wide area BS that serves macro cell deployment and local area BS that that serves pico cell deployment. For the E-UTRAN, the wide area BS is also called as the general purpose BS or macro BS.
Unlike the above BS classes, home base stations are being developed to serve even smaller and more localized areas than pico cells. Home base stations operate under licensed frequency band and are currently under standardization within both 3GPP and IEEE 802.16.
Note that in principle, the terms femto base station, home base station, home NodeB or home eNodeB may refer to a same type of base station. Presently in UTRAN and E-UTRAN the term home NodeB or home eNodeB or base station is more commonly used. The latter is more generic as it covers any type of home base station. For simplicity and consistency we will therefore use the terminology home base station (HBS) hereinafter.
In both WCDMA and E-UTRAN FDD and TDD, the HBS maximum output power (Pmax_HBS) is limited to 20 dBm for non Multiple Input Multiple Output (MIMO) case, 17 dBm per antenna port in case of two transmit antennas or 14 dBm per antenna port in case of four transmit antennas. This is normally generalized as following:Pmax_HBS=20 dBm−10*log 10(N)where N is the number of transmit antenna ports at the home base station.
The maximum output power (Pmax_HBS) comprises of the power of all downlink transmitted channels including common channels such as common pilot or reference signals, synchronization signal, control channels such as scheduling channels and data channels such as shared channel etc.
One main difference compared to other base station classes is that the HBS is owned by a private subscriber, who has the liberty to install it at any location. Thus strict network planning is not possible in case of HBS deployment. This is in contrast with other base station classes, which are deployed by an operator according to some well defined principles. The lack of precise network planning of HBSs may cause interference to other base stations, e.g. the macro BS. Due to this potential risk of interference, the maximum output power of the HBS should be regulated and controlled to minimize the impact on other base stations.
A HBS comprises of normal base station functions such as a transceiver that communicates to and from multiple User Equipments (UEs). In addition it may comprise a Measurement Unit (MU) equipped with a receiver for the purpose of performing measurements. This MU is similar to a normal UE receiver circuitry used for carrying out downlink measurements such as signal strength and signal quality of the neighbour cells (and the served cell wherein the UE is located in case of the UE performing the measurements). The measurement results may then be used to adjusting the maximum output power level. The performance requirements of the measurements done by the MU in the home base station are similar to, but not identical to, the UE measurements.
In order to distinguish the home BS radio measurements from the GPS measurements (i.e. when a GPS is integrated in the HBS), HBS radio measurements are sometimes called Cellular Radio Measurements (CRM).
As mentioned before, the HBS may operate on the same frequency channel as that of the surrounding macro BS. In this scenario mixed carrier may have to be deployed in order to offer HBS coverage. The mixed carrier scenario is obviously more challenging in terms of co-channel interference between: (1) Home base stations; and (2) home base stations and non-home BSs e.g. macro BS.
The interference situation becomes even worse in an UTRAN TDD and LTE TDD HBS deployment scenario. This stems from a fact that any difference in uplink and downlink slot or sub-frame configurations in HBS and non-HBS or within different HBSs results in severe cross-slot (or cross-sub-frame) interference. Even if the same TDD sub-frame configurations are used in all network nodes of the wireless communications network, due to the imperfect sub-frame timing due to practical constraints, there will be interference leakage.
In another scenario the HBS operates on an adjacent frequency channel to that of the surrounding macro BS (belonging to the operator which deploys the home BS). This scenario is less severe in terms of interference between the HBS and the macro BS. However, there would still be an impact of adjacent channel interference e.g. due to out of band emissions.
As stated above, a HBS comprises a MU equipped with a receiver for performing measurements over signals transmitted by other base stations e.g. non HBSs or other HBSs. This means that the HBS can perform similar measurements which are done by the actual UE. These measurements are going to be used by the HBS to perform adaptive power control i.e. maximum output power settings.
Such measurements may for example be as for Wideband Code Division Multiple Access (WCDMA) wherein three main quantities are used for mobility decisions and which may be used for adaptive power control namely (1) the Common Pilot Channel (CPICH) Received Signal Code Power (RSCP), (2) the CPICH Energy per chip to noise ratio Ec/No, and (3) the UTRA carrier Received Signal Strength Indicator (RSSI). The RSCP is normally measured by the UE on cell level basis on the CPICH. The UTRA carrier RSSI (the total received power and noise from all cells, including serving cells) is measured over the entire carrier. The CPICH Ec/No is identical to CPICH RSCP/RSSI.
In case of E-UTRAN, the following downlink radio measurements are specified primarily for mobility purpose but may be used for adaptive power control, namely the Reference Symbol Received Power (RSRP), and the Reference Symbol Received Quality (RSRQ), wherein the RSRQ is equal to the RSRP/carrier RSSI. The RSRP or the RSRP part in RSRQ in E-UTRAN is solely measured by the UE on cell level basis on reference symbols. There is no specific carrier RSSI measurement rather it is part of the RSRQ definition.
In GSM systems the following measurement is specified for mobility purpose and may further be used for adaptive power control namely the GSM Carrier RSSI.
In case of a cdma2000 1xRTT system the following measurement is used for mobility purpose and may further be used for adaptive power control namely the CDMA2000 1x RTT Pilot Strength.
In cdma2000 HRPD system the following measurement specified for mobility purpose and which may further be used for adaptive power control namely the CDMA2000 HRPD Pilot Strength.
The mentioned measurements, normally measurements on neighbour cells, are typically averaged over long time periods in the order of 200 ms or even longer to filter out the effect of e.g. fast fading. There is also an existing requirement on the UE to measure and report the neighbour cell measurements (e.g. RSRP and RSRQ in E-UTRAN) from certain minimum number of cells. For example, in both WCDMA and E-UTRAN the minimum number of cells, is 8 cells, comprising of one serving and seven neighbour cells, on the serving carrier frequency, or commonly termed as intra-frequency.
In a HBS the analogous measurements for adaptive power control may be expressed in general terms as: (1) Signal strength measured on pilot or reference signal (SS), which is analogous to UE measurements such as the CPICH RSCP in WCDMA or the RSRP in E-UTRAN; (2) Path Loss (PL) which is analogous to path loss UE measurement in WCDMA. Sometimes Path Gain (PG) is used instead, which simply is the reciprocal of PL; (3) Signal strength measured on pilot or reference signal i.e. Signal Quality (SQ), which is analogous to UE measurements such as CPICH Ec/No in WCDMA or RSRQ in E-UTRAN; and (4) Received interference (Io), which is analogous to UE measurements such as carrier RSSI in WCDMA.
Thus, the maximum output power of the HBS may be regulated and controlled based on above mentioned measurements in order to minimize an impact on other cell applications, e.g. macro networks/BSs. Typically the maximum output power will be adjusted at time intervals in the order of several seconds or even longer. Depending upon the access technology of the HBS one or more HBS radio measurements specific to that access technology may be used by the HBS to adjust its maximum output power, and its transmit power level in general. These measurements need to be combined and processed in an adequate manner to make sure that the adjusted power leads to reduction in interference to the non HBSs. At the same time, the HBS should be able to operate at relatively higher output power when the interference to the outside is limited so that HBS resources are fully exploited.
For adaptive power control based on macro BS/UE measurements, the state of the art systems typically use one or more measurements for adapting the maximum output power. However, all measurements don't have the same accuracy levels. Some solutions only use signal quality measurements for adapting the output power. The signal quality measurements, which are analogous to UE measurements on CPICH Ec/No in UTRA, or RSRQ in E-UTRA, can provide better accuracy. However signal quality measurements do not fully incorporate and depict the overall interference on a carrier. Secondly these existing solutions don't enable HBS adaptive power unit to identify uniquely the proximity of a macro network.
Another known method used for regulating or controlling the maximum output power is a smart power control method that is based on satellite system measurements i.e. satellite based methods. Global Navigation Satellite System (GNSS) is the standard generic term for satellite navigation systems that enable UEs to locate their position and acquire other relevant navigational information. Another generic term currently used for satellite based positioning is Galileo and Additional Navigation Satellite System (GANSS). Among others, Global Positioning System (GPS) is the most well known example of GNSS, and is currently in operation for more than a decade. For simplicity we will below describe GPS, however, the principles of this disclosure equally applies to any type of navigation satellite system.
GPS comprises of a constellation of 24 to 32 medium earth orbit satellites revolving around the earth. The satellites transmit pilot signals and other broadcast information, which are received and processed by GPS receivers for determining geographical positions. Signals from certain number of satellites, e.g. 5 or more, should be received in order for the GPS receiver to accurately locate a geographical position of the UE.
Assisted GPS, generally abbreviated as A-GPS, is a system which can improve the start-up performance of a GPS satellite-based positioning system. It is used extensively with GPS-capable cellular phones, by taking advantage of the capability of the cellular network to provide the precise time, orbital data or almanac for the GPS satellites, etc. enabling the GPS receiver to lock to the satellites more rapidly. Among various positioning methods, A-GPS is considered to be one of the most viable and commonly used one.
The HBS may for example comprise an A-GPS receiver or simply a GPS receiver, or support other possible positioning method/system. Nevertheless in a legacy network i.e. a network comprising non HBSs, supporting GPS services, the non HBSs may also comprise a GPS receiver. The GPS receivers are normally used to provide some basic GPS related information e.g. detectable satellites, base station GPS coordinates etc, to the UEs. This GPS related information assists a UE in determining a GPS position relatively quickly especially after a cold start e.g. initial access to a network or coming out of a tunnel.
In order to receive a GPS signal with relatively good quality, a GPS receiver needs to have a Line-of-Sight (LoS) radio link to GPS satellites. However, when the GPS receiver is located inside a building, LoS connection between the GPS receiver and the GPS satellites may not be possible. Therefore, weak GPS signal may be received. One approach to cover indoor GPS users is to deploy a use of GPS repeaters. A simple GPS repeater simply receives the GPS signal via outdoor antenna deployed on top of a building, and then amplifies and retransmits the received GPS signal inside the building. Normally, GPS repeaters should only cover areas where GPS signals are unavailable or too weak, in order to avoid messing with the real GPS signals. GPS repeaters are being deployed in tunnels, large buildings etc. Especially in the case of large buildings, the HBS may also be used for providing indoor coverage for UEs. However, there may be overlapping zones where both GPS signals and repeated GPS signals are received.
Two types of information can be exploited from the GPS receiver, namely the number of detected satellites (NS) and the reception quality (QR) of the detected satellites. In a normal environment, at least 4-5 satellites should be visible with sufficient quality to obtain good accuracy of a geographical location. Furthermore, the reception quality can be an aggregate value of all the detected satellites e.g. weighted average of all detected satellites or certain number of strongest satellites. Herein, we refer NS and/or QR as GPS detection performance metrics or criteria.
Depending upon the GPS detection performance, the maximum output power of a HBS may be set according to different mapping functions. Any suitable mapping functions such as weighted sum or average of detected satellites (NS) or of their reception quality (QR), or of both, may be used to create lookup tables with multiple maximum output power levels for a HBS.
The mapping function, which maps the GPS detection performance to the maximum output power of the HBS (Pmax_HBS) could use either NS or QR or combination thereof as expressed in (1), (2) and (3):F(α1NS)→Pmax—HBS  (1)F(α2QR)→Pmax—HBS  (2)F(α1NS,α2QR)→Pmax—HBS  (3)Where: α1 and α2 are the weighted factors. Any suitable mapping function such as weighted sum or average can be used.
The above mapping functions are used to create lookup tables to generate the maximum output power based on NS or QR or combination thereof.
A poor GPS detection performance corresponds to a scenario where the HBS is isolated and shielded from outside/other base stations. This means less interference is generated by the HBS e.g. to the outside macro BS. Therefore higher maximum output power can be used in order to improve the HBS coverage and performance indoor. On the other hand, a good GPS detection performance indicates that the location of the HBS may cause significant interference to the outdoor network and therefore lower maximum output power should be used at home base station in order to protect the Macro UE.
The above mentioned smart output power control method based on GPS measurements i.e. detection performance, may avoid some problems which may arise with adaptive power control. However, the smart output power control method will not work in scenarios where GPS repeaters are deployed inside the premises. This is because in such scenarios, strong repeated GPS signals are always received no matter where the HBSs are located indoors. Therefore it is almost impossible to decide whether the HBS is isolated and adjust the maximum output power accordingly.
Thus, both adaptive power control based on macro BS/UE measurements and smart output power control based on GPS measurements on the one hand are unable to fully protect the macro network as HBSs may be operating at higher output power than desired. On the other hand the maximum output power may be conservatively set causing poor coverage and performance loss of HBS.